This treatment of human aging draws on biomedical research and the natural history of animals and plants to describe this natural phenomena in detail, helping the reader to understand this complex process. In the aging process of humans and many other species, the authors find some answers to why aging must exist at all. They present contemporary theories of aging and their implications for the prospect of significantly extending the human life span. The "hows" of aging, including genetic mutations, cellular degeneration, wear and tear on the body, and the gradual weakening of the body's immune system are explored carefully and fully, as are the genetic and environmental causes of aging.